Grommash never fights alone if he can help it. Every second breath is a war-cry to whatever still stands near him - and the more of his herd answers, the harder he swings back.
Up to 3 nearby allies get a short attack/damage buff.
His own attack does bonus damage scaled by how many allies are CURRENTLY carrying the Herdcall buff - the first mechanic here that rewards allies being alive and buffed, not dying.
His epic spell answers by raising two ancient temple guardians (reflavored bele_mumy) to reinforce the herd.
Karrion Temple had no round-1-through-8 boss at all - its real ceiling was the Cursed Swordsman at CR16 (a plain monster entry, not a built PRC boss); Grommash is pitched one notch above at CR17, into 6 free Sparky slots, the most headroom of any target this round.
He is the bestiary's first Orc Warlord - reflavored as a bound minotaur herd-warlord rather than an orc chieftain, cloned from red_minotaur.
Real Epic Spell: Mummy Dust (id 637/874) - Karrion Temple had no epic-spell boss yet, and the temple's own existing undead roster (bele_mumy, bele_mumwar) made reusing that exact blueprint for the summon the obvious, honest choice rather than a new one.